An integral part of any communication system is the protocol that is utilized to properly transmit the desired information from a first location to a second location. As an increasing amount of information is transmitted through optical fiber communication systems, numerous standard protocols have been established and more are currently being defined. These protocols utilize different rates and formats in order to balance the advantages of increased flexibility and services with the complexity and overhead that comes as a result.
For example, there are synchronous standards such as SONET in North America and SDH in Europe, numerous other continuous formats, and numerous burst formats. Burst formats do not have a continuous clock, but transmit bursts of data without requiring any given phase relationship between bursts. The phase of the clock in continuous formats has continuity under normal conditions.
For each of these protocols, transponders, regenerators, and multiplexer/demultiplexer systems have been developed for the particular bit rate and conditions that apply. These components are designed specifically for the particular protocol that it is to function with and cannot generally be used for other protocols.
To allow interfacing between systems that utilize different protocols, mapping devices have been developed to transfer data information within one protocol into a format that can be used within a system of a different protocol. The key to these mapping devices though are that they are specific to transferring one protocol into one other protocol and cannot generally be used with any protocols that they are not specifically hardwired for. For example, Bellcore Generic Requirement 0253 (GR-0253) describes in detail the standard mappings of the common asynchronous transmission formats (DS0, DS1, DS2, DS3, etc) into SONET. Similar mappings are defined for the ETSI hierarchy mapping into SDH.
The key to these mappings are that they are each very precisely tuned for the particular format and bit rate that is being mapped, plus or minus a tolerance such as 20 parts per million (ppm) on the bit rate. This means, that using these standard mappings, a signal that has a bit rate even 1% different than that of a DS3 format cannot be transported within a SONET system. A different hardware unit is generally required to perform the mapping of each kind of signal.
These limitations on standard mappings become even more pronounced when considering the use of a multiplexer that may have more than one protocol among the input signals and an output signal of yet another protocol. Similar problems can be seen with the use of a demultiplexer. Very specific multiplexers have been developed that perform multiplexing functions for a limited number of protocols. For instance, there is a multiplexer that can combine signals in the OC-3 and OC-12 formats to generate an output signal in the OC-48 format.
The key limitation to the current components used for multiplexing and demultiplexing is that a different piece of hardware is required for each different protocol or set of protocols that are to be combined. This is going to require a substantial number different mapping devices and protocol specific multiplexers/demultiplexers as the number of protocols continue to increase with new components being required with the advent of each new protocol.